TP-1 Casting for Mg-Al Binary Alloys

Pure magnesium and Mg-Al binary alloys with various concentrations of aluminium (0.1, 0.3, 1, 3, 9wt%) have been cast following typical TP-1 test procedures with 10 K superheat and melt shearing, and at 10K, 30K, and 50K superheat without melt shearing. Figure 4.2.1 shows the microstructures of the cross sections of pure magnesium and Mg-Al binary alloys with 10 K superheat. In this figure, the only difference between the samples is the composition of aluminium. As it is shown, the weight percentages of aluminium contained in the alloys have significant influence on the final grain structures. The linear intercept method has been applied to measure grain sizes with these images. The results show a significant decrease in grain size with the increased concentration of aluminium. The measured linear intercept length of Mg-Al binary alloy on cross sections have been plotted against composition of aluminium and 1/𝑄 in Figure 4.2.2 and 4.2.3, respectively. As shown in these two figures, the measured data has an inverse relationship against composition and a linear relationship against 1/𝑄. Therefore, it appears that the quantity of aluminium does have a certain influence on grain sizes of magnesium alloys, as it has been stated in Easton and StJohn’s model [18, 52, 53]. There is one thing that needs to be noted, which is that all thedata is measured from the intercept lengths of the cross sections.

Figure 4.2.1 Microstructures of cross sections of Mg-Al binary alloys without shearing showing

the effect of Al contents on grain structure: (a) pure magnesium cast at 660 ºC; (b) Mg-0.1wt%Al cast at 659 ºC; (c) Mg-0.3wt%Al cast at 659 ºC; (d) Mg-1wt%Al cast at 658 ºC; (e) Mg-3wt%Al cast at 645 ºC; (f) Mg-9wt%Al cast at 611 ºC. For each alloy, the pouring temperature is 10 K above the liquidus of the alloy.

(a) (b)

(c) (d)

Figure 4.2.2 Measured linear intercept lengths of Mg-Al binary alloys on cross sections as a

function of composition of aluminium, showing the effects of aluminium content, superheat, and intensive shearing on grain size.

Figure 4.2.3 Measured linear intercept lengths of Mg-Al binary alloys on cross sections as a

function of 1/𝑄, showing the effects of aluminium content, superheat, and intensive shearing on grain size.

There seems to be no reason to doubt that these measured linear intercept lengths are the actural grain sizes. However, when the microstructures of the vertical sections are shown, the linear relationship between 1/𝑄 and the so-called ‘grain sizes’ might be questionable. As shown in

Figure 4.2.4, on the cross section, all the grains seem to be equiaxed grains. However, in the

vertical sections, it is easy to see that they are actually columnar grains. Therefore, in order to determine whether the measured linear intercept lengths are grain sizes, both cross sections and vertical sections need to be examined. The measured linear intercept lengths can be considered as grain size only when the microstructures are equiaxed grains in both cross section and vertical section.

Figure 4.2.4 Schematic illustrations of TP-1 casting sample and the microstructure of the sample.

A good example is shown in Figure 4.2.5. This figure is a comparison of the microstructures of Mg-1wt%Al binary alloy vertical sections cast at 10 K superheat with and without shearing. It is easy to find that the sample in Figure 4.2.5(a) contains columnar grains, while the one in Figure

4.2.5(b) contains only equiaxed grains. Therefore, the measured linear intercept length of the

cross section of the sample in Figure 4.2.5(a) can’t be simply considered as grain size. In fact, it is not even the columnar width but a value with no clear physical significance. A comparison between such values is completely meaningless. It is then proposed that for TP-1 samples, both the cross section and vertical section of these samples need to be examined before any measurement has been made.

Figure 4.2.5 Microstructures of Mg-1wt%Al binary alloys vertical sections cast at 658ºC (10K superheat): (a) without shearing, and (b) with shearing.

Figure 4.2.5 (a) is not the only case. Most data with low aluminium content (less than 1 wt%) fits

in such a situation. Therefore, the data can only be regarded as incorrect data rather than grain sizes. As shown in Figure 4.2.6 and 4.2.7, only the data outside the dashed box are actual grain sizes.

Figure 4.2.6 Grain sizes of Mg-Al binary alloys as a function of composition of aluminium,

showing the effects of aluminium content, superheat, and intensive shearing on grain size.

(a) (b)

Figure 4.2.7 Grain sizes of Mg-Al binary alloys as a function of 1/𝑄, showing the effects of

aluminium content, superheat, and intensive shearing on grain size.

In Figure 4.2.6 and 4.2.7, the data in the dashed boxes are not actual grain sizes. Therefore, the inverse relationship between grain size and composition is debatable and the linear relationship between grain size and 1/𝑄 doesn’t seem to exist for Mg-Al binary alloy. Table 4.3 shows the grain sizes for Mg-3wt%Al and Mg-9wt%Al under various casting conditions. The differences between grain sizes under the same condition are believed to be caused by composition. Since such differences are relatively small, it is suggested that the major function of solute is to cause columnar to equiaxed transition (CET), after CET, solute has little effect on grain size.

Table 4.3 Grain size for Mg-3wt%Al and Mg-9wt%Al with various casting conditions.

Al (wt%)

10K Superheat 30K Superheat 50K Superheat

With Shearing Without Shearing Without Shearing Without Shearing

3 93.13968 106.8601 139.6719 230.3492

9 66.83963 115.7882 108.9663 189.3461